RFR: Clarified backport docs [v2]

[Iris Clark]

On Wed, 20 Aug 2025 19:07:49 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Explaining the difference between jdk21u and jdk21u-dev.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed bug

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/guide/pull/158#pullrequestreview-3137960635